A conventional optical lithographic stepper system, for imagewise exposure of a coating of resist on a semiconductor wafer, includes a light source for emitting a beam of actinic radiation directed towards the wafer, an imaging lens for imaging the light source on an exposure mask (also called a reticle) that defines features that are to be transferred from the mask to the resist coating, and a projection lens for imaging the mask on the resist coating. Ideally, every point of the mask is either opaque or transparent and, subject to diffraction limitations, the lithographic system results in the resist coating being exposed in regions that correspond to transparent regions of the mask and being unexposed in regions that correspond to opaque regions of the mask. The resist is then developed, leaving a pattern of resist features that corresponds to the pattern of opaque features of the exposure mask (in the case of a positive resist), and the underlying wafer is selectively etched using the patterned resist to protect the wafer. FIG. 1 illustrates this ideal mode of operation. As shown in FIG. 1, resist features 4 are images of mask features 2. For simplicity, projection optics between the mask and the wafer, and the image reduction effected by the projection optics are not shown in FIG. 1.
In operation, the optical lithographic stepper system effects stepwise relative movement of the exposure mask and wafer transverse to the axis of the system so that different sites or fields of the wafer can be exposed through the mask. A field or “image field ” is a region that is exposed without moving the wafer or the mask with respect to the lens; or in the case of a stepper-scanner instrument, a field is a region that is exposed in one, linear, continuous scanning motion of the wafer and mask stages. Stepper-scanners project a slit-shaped region, typically 26 mm by 8 mm, on the image plane (wafer). A field is exposed by scanning the slit shaped image in a direction that is parallel to its short dimension. The maximum field size on the order of 26 mm by 33 mm.
All imaging systems suffer from some amount of flare (an effect that mixes light from one part of the image with light from another part). Referring to FIG. 2, flare results in the resist features 4′ being an imperfect match for the mask features 2. Flare degrades system performance, causes CD (critical dimension) variation, and decreases process latitude. Flare may be caused by particles or irregular films deposited on optical components, surface roughness caused by grinding lens and mirror elements, density variations in lens blanks, birefringence of lenses, imperfect antireflection coatings, imperfect absorbing coating on the lens mounts and lens barrel, multiple reflections between the wafer and the mask, multiple reflections between the wafer and the lens elements, among other factors.
Referring to FIG. 2, light that is issued from point O on the mask is imaged to point O′ on wafer 6. In the absence of flare, the intensity at O′ would be I0. In the presence of flare, a portion I0g(r)dA of the intensity is misdirected to point O″ from an infinitesimal neighborhood of O′ of area dA.
The function g(r) is referred to herein as the aggregate flare density function (or aggregate flare point spread function). The flare density function depends on the distance r=|O′O″|. between the source and observation points. Measurements that have been made indicate that the aggregate flare density function decreases monotonically from a peak value as r increases.
Because flare represents imperfect behavior of an optical lithographic system, it is desirable to reduce the flare of the projection optics by good optics design and maintenance. However, some residual flare is inevitable even in the highest quality optics. The remaining flare can be managed by taking flare into account in the design of the mask. The patterns on the mask can be compensated for flare and other optical imperfections. Compensating the mask for flare requires the knowledge of the flare density function.
Some prior art measure flare in an optical lithographic stepper system by using a photoresist detector 5. Such a detector comprises a wafer 6 or other substrate having a coating of photoresist 7 on one surface. The photoresist is exposed in the stepper system using a test structure as an exposure mask and is subsequently developed. Either the presence or the dimensions of features of the mask, as transferred to the photoresist, are measured and these measurements are used to characterize the flare of the stepper system.
FIG. 3 illustrates a photoresist detector 5 and a reticle 12 that is transparent except for an opaque feature 14 of width W. Different sites on the photoresist detector are exposed through the reticle at several progressively increasing doses. By exposing the detector at different sites at different exposure doses, we can determine the dose that is sufficient to clear the photoresist at locations remote from the opaque feature 14 (projected to wafer level) and the dose that is required in order to clear the resist within the geometric image of the opaque feature 14. Geometric image means: the image of an object that would form in the absence of flare and diffraction. It is a hypothetical image of the object that is obtained by ray tracing. Let us assume that an exposure dose D0 is just sufficient to clear the resist at locations that are remote from opaque reticle features. If there were no flare, the region within the geometric image of the feature 14 would remain unexposed (except for diffraction and aberration effects) regardless of dose. Due to flare, however, some proportion of the light from transparent regions of the reticle reaches the detector within the geometric image of the opaque feature 14. If the dose required to clear the geometric image of the feature 14 is D1, we can define the flare level F at a distance W/2 from a linear boundary of an opaque feature by the ratio D0:D1.
Flagello, D. et al., “Optimizing and Enhancing Optical Systems to Meet the Low k1 Challenge”, Proc. SPIE, vol. 5040, pp 139-150 (2003), discloses that scattered light in an optical lithography system may be measured using a reticle that is transparent except for opaque square pads of several different sizes. A photoresist detector is exposed through the reticle at several progressively increasing doses. The minimum exposure dose to clear each pad is determined.
Kirk, J. P., “Scattered Light in Photolithographic Lenses”, Proc. SPIE Vol. 2197, p. 566-572 (1994), discloses that flare may be measured by observing the extent to which an edge of the unexposed photoresist has receded from the corresponding edge of the geometric image of an opaque feature.
Prior art that measure flare by exposing a photoresist assume a functional form of the flare density function. A Gaussian density function is frequently assumed. The functional form of the density function is not directly obtained from the measurements.
High-order wavefront aberrations contribute significantly to total flare. Matsuyama, T. et al., “Nikon Projection Lens Update”, SPIE 2004, and M. Kerkhof et al., “Full Optical Column Characterization of DUV Lithographic Projection Tools”, SPIE 2004, disclose using an interferometer to measure these aberrations. High-order wavefront aberrations only yield short-range flare.
Conventional methods of measuring flare measure only the aggregate flare. Optimal management of flare would require information regarding the distinct characteristics of short, medium-range (field-scale) flare, and long-range (wafer-scale) flare, respectively.
Flare may also be asymmetrical with regard to both location in the field and angle of incidence on the wafer. For example, the flare may be higher at the left of the field than at the right; and at a given point of the field flare incident from the right may be stronger than the flare incident from the left; and flare may scatter preferentially in one direction causing the point spread function to be not circularly symmetric. Priort art described above do not permit characterization of asymmetry of flare.